phase variation
A mechanism of gene regulation in which a gene is switched on in response to an environmental condition in some of the cells in a population leading to a mixture of expressing (phase ON) and non-expressing (phase OFF) cells. An example of this type of regulation is seen in the expression of type 1 fi mbriae (an adhesin) in E. coli. Transcription of a group of genes (fi m genes) is controlled by a short invertible element in the promoter. Expression of the gene cluster is either ON or OFF, depending upon the orientation of the invertible element.